The melting temperatures of the fluorides of the alkali metals and alkaline eart

Question

The melting temperatures of the fluorides of the alkali metals and alkaline earths are listed below. Consider the observed variation in terms of both ionic charcter of the bonds and the ionic potentials. How should the melting temperatures vary according to Goldschmidt's rules? Which compounds conform to theis prediction and which do not?

Compound:    Melting temperature degrees C:

LiF        845

NaF       993

KF         858

RbF        795

CsF         682

BeF2      800 (sublimes)

MgF2      1261

CaF2        1423

SrF2       1573

BaF2        1355

Explanation / Answer

According to Goldschmidt's rules the melting point of a crystal lattice vary according to the compatibility of the cations and anions forming it as per the rules-

1)Similar size of the cation and anion,difference less than 15%.this makes them fit better in the crystal

2)Charge difference should not be only in case of coupled substitution of the ions ,such that electroneutrality principle is followed. More than +1 unit of difference is less applicable

3)Ionic potential-This is infact the ratio of ionic charge to the ionic radius.If it is higher ,then the density of electrical charge on the surface of ion is high and stronger is the ionic bond. (for eg ionic potential of K+ is less than Sr2+)

4) higher electronegativity difference between the cations and anions is favourable and forms stronger bonds.Subtituting cation should have compable electronegativity to the cation in the crystal.

The stronger the bonding interaction between the cation and anion in a crystal lattice based on the given properties, higher the melting point.

Higher ionic character of bonds corresponds to the ionic potential of the cations.Higher the charge by size (smaller the size) ratio of cation, more strongly ionic(more polarizable, by fajan's rule) it is and strongly bonds, so higher is the melting point.

From the following compounds-

Li+ has the highest ionic potential ,followed by Na+,K+,Rb+,Cs+ so is their ionic character as size decreases down the group.So m.pt must decrease in the order of their ionic potential. NaF ,KF is not following this trend.

Also ionic potential of alkaline earth cations are higher than the corresponding alkali metals due to smaller size of the former. and high charge+2. Also increase of size down the group decreases ionic potential and so m.pt

in alkaline earth metals, BeF2 has highest ionic potential due to smaller size but it sublimes.not following the trend.Rest MgF2,CaF2,SrF2, also does not varies acoordingly.

BaF2 again follow the trend as its m.pt is less than SrF2 as per the ionic potential values

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